1. Field of the Invention
This invention relates to a hot isostatic pressing (HIP) method for densifying a metal or ceramic porous body by subjecting it to a high pressure, high temperature gas.
2. Description of the Prior Art
The HIP method is a technique to press a body to be treated isostatically using a high pressure, high temperature gas as the pressing medium. It is known to prepare a dense sintered body containing few pores by treating a porous body such as a metal or ceramic powder sealed in a capsule or a sintered body of a powder by HIP. Heretofore, the optimum HIP conditions to achieve the densification of a porous body were determined by repeating HIP treatment with changing the treating conditions. Each treating condition was evaluated by measuring the density and, if necessary, further incorporating the observation of the texture and the measurement of the strength. Such a method was troublesome, requiring labor and time.
In order to reduce the trial and error times and to determine the optimum HIP conditions efficiently, McCoy et al. devised a special HIP apparatus including a dilatometer to measure the volume change of a sample during HIP treatment (Am. Ceram. Soc. Bull., vol. 64, No. 9, pp 1240-1244, 1985). In the HIP apparatus, a sample table and a probe of the dilatometer is set in the pressurized heating space. The probe is connected with a differential transformer set at a low temperature portion on the outside of the space. When a test piece is put on the sample table, the volume change of the test piece is transmitted from the probe to the differential transformer to detect the expansion or contraction of the test piece by the output. In the HIP apparatus, the subject to be measured is the dimensional change of a test piece. McCoy et al. used a column-shaped alumina molded body sealed in a stainless steel capsule as a test piece, and measured the variations with time of the expansion or contraction quantity of the test piece in various pressure elevation and temperature elevation patterns by this apparatus. Based on the measured results, the pressure and temperature necessary for the densification of the alumina molded body were determined. The determined conditions were applied to the HIP treatment of a big alumina molded body, and a suitable HIP treatment was made possible without repeating trial and error. However, in the above conventional method using a dilatometer, it is necessary to repeat HIP treatment at least twice, i.e., one HIP treatment of a test piece and the HIP treatment of the object to be treated.